As is well-known in the metalworking Art, laser cutting is normally performed by focusing a laser beam on the metallic workpiece which is to be cut. At the same time, a cutting gas, normally oxygen, is blown onto the workpiece through a nozzle. The principle of laser cutting equipment will be apparent from a subsequently presented description of Drawings appended to this application. A laser beam from, for example, a CO.sub.2 laser is focussed by a lens through a nozzle device onto a workpiece, for example a metal sheet. The cutting gas, oxygen, is led through an intake conduit to a plenum chamber and is forced coaxially with the laser beam through a nozzle mouth out towards the workpiece. The nozzle device is disposed in a support member in which bearing balls are journalled, against which the workpiece--the metal sheet--abuts. A support member is disposed against the underface of the metal sheet and is provided with a hole under the nozzle device. The metal sheet moves, during the cutting operation, in a given direction and may, during this movement, be mounted on, for example, a mobile co-ordinate cutting table.
The purpose of the cutting gas--the oxygen--is twofold: (a) to protect the lens 2 in the cutting equipment from the spatter and slag created during the cutting process, and (b) to flush out molten material and slag from the kerf which is formed as a result of the cutting operation. In this operation, the molten material and slag 12 are flushed out through the hole 10 in the support member 9. If the sheet 6 is of carbon steel or of stainless steel, the oxygen has a further purpose to fulfil, namely to react chemically with the steel and thereby generate heat, which facilitates the cutting process. Thus, apart from melting of the sheet by the action of the laser beam, laser cutting also entails a steel combustion of the type which occurs in traditional gas cutting.
In the prior art a cutting gas has been used which consists of up to 100% oxygen, since such a cutting gas gives the best results with respect to attainable cutting speeds and acceptable kerf qualities. The quality contemplated here is dependant upon several parameters among which are cutting speed, cutting gas pressure (i.e. the gas pressure in the nozzle), nozzle diameter (i.e. the hole diameter in the nozzle mouth), nozzle spacing (i.e. the distance between the nozzle mouth and workpiece) and finally laser output. In general terms, the increased cutting gas pressure gives increased cutting speed. However, the cutting gas pressure must be limited in view of such factors as the use of focussing lens. Laser cutting with pure oxygen as the cutting gas involves, however, certain drawbacks, in particular in conjunction with the cutting of stainless steel. Oxides are formed during the melting of the material in the workpiece. These oxides, together with molten material, are blown out through the kerf. A portion of the oxides and the molten material is, however, deposited as burrs on the underside of the kerf. These burrs may, in particular in high-alloy steel, be difficult to remove. In cutting with pure oxygen as the cutting gas, the melted zone will be mixed with slag, i.e. a mixture of oxides from the workpiece. The slag flakes in the kerf may give rise to problems in subsequent welding. A metal sheet which is cut with pure oxygen as the cutting gas may, thus, give a welding joint which contains slag pockets which may be difficult to remove. One method of avoiding the difficulties inherent in oxide formation and consequential burr and slag formation would be to replace the oxygen in the cutting gas by an inert gas. However, this would result in a decline of the cutting speed to a very low rate. Hence, such a process is beset with serious disadvantages.